This specification relates to devices and techniques of coupling radiation energy to a quantum-well radiation sensor.
An infrared quantum-well semiconductor sensor usually includes a quantum-well structure formed of alternating active and barrier semiconductor layers. Such a quantum-well structure can have different energy bands. Each can have multiple quantum states. An intraband transition between a ground state and an excited state in the same band (i.e., a conduction band or a valance band) can be used to detect infrared (“IR”) radiation by absorbing IR radiation at or near a selected resonance IR wavelength. The absorption of the radiation generates electric charge indicative of the amount of received radiation. The radiation-induced charge can then be converted into an electrical signal (e.g., a voltage or current) to be processed by signal processing circuitry.
The compositions of lattice-matched semiconductor materials of the quantum well layers can be adjusted to cover a wide range of wavelengths for infrared detection and sensing. In comparison with other radiation detectors, quantum-well structures can achieve, among other advantages, high uniformity, a low noise-equivalent temperature difference, large format arrays, high radiation hardness, and low cost. Infrared quantum-well sensing arrays may be used for various applications, including night vision, navigation, flight control, environmental monitoring.
A quantum-well infrared sensor usually responds to incident radiation with a polarization that is perpendicular to the quantum well layers, i.e., parallel to the growth direction. This is because this polarization can induce an intraband transition at a desired infrared wavelength. In applications based on imaging at focal plane arrays, the photodetector array is often oriented perpendicular to the scene to be imaged. Since the electric vector is essentially parallel to the quantum well layers in this arrangement, the quantum well layers absorb little or no light.
Hence, a coupling mechanism is often implemented to couple incident radiation in a way that at least a portion of the incident radiation becomes absorbable by the quantum well layers. The coupling efficiency of the coupling mechanism can be characterized by the percentage of absorbable radiation, but not what is actually absorbed, of the total incident radiation. One way to provide proper coupling is to use random reflectors or corrugated surfaces to scatter a portion of the incident radiation into the correct polarization for absorption. Alternatively, one-dimensional or two-dimensional grating couplers can also be used to convert normally-incident radiation to waves have components that propagate along the quantum well layers.
However, the coupling efficiencies of these coupling schemes are sensitive to the wavelength of the radiation. Hence, their applications are limited to detection of radiation at a single selected wavelength or a narrow spectral range and hence cannot be used for detection of radiation of multiple colors. In addition, these coupling schemes direct only a portion of the incident beam to be parallel to the absorbing quantum well layers and hence limit the coupling efficiency to an upper limit that is determined by the percentage of the portion of radiation propagating parallel to the quantum well layers.